The surface of Ryugu is covered in boulders, with no smooth locations providing a completely clear landing site. Boulders range from 3 meters in diameter (the lower limits of detection at this resolution photography) to over 10 meters in diameter, with larger boulders increasing in density near the poles. (JAXA)

The surface area at the candidate touchdown sites for Hayabusa2 range is between 10 and 20% covered in boulders. The primary landing site is L08, and the backup sites are L07 and M04. (JAXA)

This composite spectral slope map shows the surface topography (greyscape) and spectra (colour). The spectra has been exaggerated for clarity. Red is interpreted as more weathered geological materials, and blue is interpreted as fresher materials. The surface is heterogenous, with slightly more fresh material near the equator (primary landing site L08 and backup site L07) and a higher concentration of weathered materials at mid-latitudes (backup landing site M04).(AXA / University Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / University of Aizu / AIST / CNES / DLR)

Primary (L08) and backup (L07 and M04) landing locations for the initial touchdown of the Hayabusa2 spacecraft. (JAXA)

MASCOT’s landing ellipse is complicated by bouncing, so the science team ran between 10,000 and 100,000 Monte-Carlos trials to model potential final resting locations. (JAXA / University Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / University of Aizu / AIST / CNES / DLR)

Landing site candidates for MASCOT (MA-9), MINERVAL-II (N6), and the primary (L08) and backups (L07 and M04) initial touchdown locations for Hayabusa2 on the asteroid Ryugu. (JAXA)

IAU 2018 delegates remotely watching this morning’s press conference announcing initial Hayabusa2 landing site candidates. Early this morning and halfway around the world, the Japanese Space Agency (JAXA) announced the initial landing sites for their asteroid sample return mission. Here in Vienna, we watched remotely, accompanied with in-person commentary from Masaki Fujimoto, Deputy Director General of JAXA’s Institute of Space and Astronautical Science.

This was very much a Japanese moment, and, to bring us a literally bigger picture, the National Astronomical Observatory of Japan (NAOJ) booth (0-20) kindly lent their larger screen and comfortable seating for the remote press conference. Fujimoto also provided English-language slides to accompany the presentation, providing translations and answering questions for IAU delegates.

After cruising for 3.5 years, the Hayabusa2 mission arrived at the asteroid Ryugu earlier this summer. Since then, it’s been making detailed surface observations in anticipation of releasing the MASCOT and MINERVA-II landers, and the spacecraft Hayabusa2 making its first of several planned landings.

MASCOT is a French / German lander capable of tumbling to reposition itself on the surface while investigating surface structure, mineral composition, thermal behaviour and magnetic properties. MINERVA-II houses tiny hopping octagonal “rovers” loaded with a stereo camera, wide-angle camera, thermometer and accelerometer. They carry optical and ultraviolet LEDs to light up floating dust particles knocked free by their bouncing. Meanwhile, the Hayabusa2 spacecraft will repeatedly land to survey and sample various locations on the asteroid over the next year and half, departing in December 2019 and reaching Earth a year later.

The primary considerations for selecting landing sites are spacecraft safety and scientific interest of each site. “In practice for this situation it was the safety first, because scientifically it turns out that any spot on the surface of the asteroid is equally valuable,” Fujimoto explains. “We learned that that asteroid is not friendly to us, so [landing is] not as easy as we had supposed when we were planning the mission.”

The surface of Ryugu is not exactly like the science team expected. “Somehow, we thought that Itokawa is a template for surface conditions for an asteroid,” laughs Fujimoto. Instead, Ryugu has a rough surface everywhere without smooth flat patches, is darker than anticipated, and may have fewer hydrated minerals.

The roughness is a problem when picking a landing site, as the team was unable to locate any 100 meter square patch that was clear of boulders 3 meters in diameter or greater. Boulder distribution is not uniform, with a slightly higher concentration of larger boulders closer to the poles and smaller boulders near the equatorial ridge. Of the initial landing sites under consideration, none of them had less than 10% of the surface covered in boulders, with some up to 20%. This mean that no site is perfectly safe, so the team will need to trade off risks in determining the initial site.

An initial analysis of Ryugu’s surface materials is coming from analyzing the spectra, how much sunlight of which wavelengths are being absorbed and reflected. Albedo is a measure of how much sunlight is reflected, ranging from 0 for pitch black to 1 for a perfect mirror.

“The albedo is super low. It’s like 0.02!” says Fujimoto of the asteroid’s unusual dark surface. For comparison, the only material of similar non-reflectivity on Earth is charcoal, with an albedo of 0.04. “There’s no counterpart in meteorites, so whatever sample we get must be something new.”

The team is also looking at the wavelengths of light reflected. When exaggerated to show variation more clearly, Ryugu is a speckled mix of reds and blues, which the team is interpreting as material that has been weathered more by exposure to space (red) and fresh material with little weathering (blue). Although the entire asteroid has a mix of both materials, the fresher material is slightly more concentrated along the equatorial ridge, making landing sites near the equator a higher scientific priority in order to collect the least-altered rock samples.

Hayabusa2 scientist: The surface is uniform not in it’s all the same everywhere, but it’s got the same mix of materials everywhere.Me: So like this granite floor?Scientist: Yes!

Science teams for each of the spacecraft — the main Hayabusa2 spacecraft and the smaller MASCOT and MINERVA-II landers that hitched along for the mission — met independently to mull over the data to select tentative landing sites. The discussions went smoothly, thanks to intense preparations during the spacecraft’s multi-year cruising from Earth to the asteroid belt.

Hayabusa2’s scientists appointed a member of their team they nicknamed “the god” who was responsible for creating an imaginary asteroid model for practice sessions. “He created the asteroid, and he didn’t tell anybody the answers,” explains Fujimoto. “Everybody had the data that was created by the god.” During these practice discussions, the science teams for each spacecraft eventually figured out which criteria was most important for their decision of landing site. “Using that pseudo-data, all the discussions and procedures have been run through once or twice already,” says Fujimoto. “When we got the real data, people knew what features to look for, what features are critical for the outcome of that landing site selection.” For the Hayabusa2 team, that discussion was so streamlined it only took a single day to narrow down the sites to an initial choice and backup (although that meeting did run three hours longer than scheduled!)

“Scientists try to talk about everything, but for that landing site purpose, this is the critical element that you should be talking about,” says Fujimoto of how the discussion stayed so focused despite scientists’ natural inclination to curiosity. “Everybody was going into the same direction.”

For now, it looks like Hayabusa2 will make its first touchdown near the equator of the 865-meter diameter asteroid. Meanwhile, the MINERVA-II and MASCOT landers will set down closer to the poles, one in each hemisphere, although their bouncing makes for a more complicated and larger landing ellipse predicted with thousands of simulated scenarios. By chance, the three landing site candidates do not overlap, reducing the chance that one spacecraft will muddle up science operations for another.

Of course, even with the initial site selections made, that doesn’t mean things can’t change in the future.

“Even though we have multiple events, they are not independent,” explains Fujimoto. “Everything is converging towards a successful touching down and sampling from the surface. That’s the way I look at it.” Every time Hayabusa2 ducks closer to the surface, it’ll keep taking more high-resolution photographs for the team to analyze for scientifically interesting sites or new hazards. Similarly, any new data from the MINERVA-II and MASCOT deployments will also feed into continuing to analyse and re-analyse potential landing sites for Hayabusa2 and additional MINERVA-II deployment locations. This may ultimately delay the initial landing past its tentative autumn date.

“I think that’s not a bad thing if [the initial landing] doesn’t happen at the end of October because it only means that we are careful enough,” says Fujimoto. “We are not going to do it hastily.”

These efforts go beyond the immediate Hayabusa2 and MINERVA-II teams at JAXA and the MASCOT teams at the French (CNES) and German (DLR) space agencies. NASA and ESA are both providing extensive infrastructure support with the use of the Deep Space Network antennas, which can transfer data at a higher rate and provide global coverage that JAXA would be unable to do alone. “Without the help from NASA and ESA , and without having that K-band communication system alive and used in a practical way, I don’t think we would have had this much data in a timely way,” says Fujimoto. “Getting the data in a timely way, and then processing the data in a timely way, really led to this discussion in making the selection of the landing site.”

This type of support isn’t flashy and doesn’t garner much media attention, which makes Fujimoto even more grateful for the international support for this mission. “Getting help in the infrastructure, that’s not very heroic,” says Fujimoto. “It’s not like standing on the stage telling people ‘Hey, I did this, I did that!’ It’s more really understanding the necessity and really understanding what’s needed to make a mission happen in a successful way.”

Although it wasn’t brought up during the press conference, another more subtle form of cooperation is that JAXA and NASA have set up a sample sharing agreement between their two asteroid sampling missions, Hayabusa2 to Ryugu and OSIRIS-REx to Bennu.

It’s this sort of cooperation and putting the science and mission success first that Fujimoto keeps emphasizing. “It’s this kind of mutual trust and mutual understanding that I really appreciate,” he says. “The discussion went very smoothly because people weren’t behaving in a very egoistic manner.”

That mutual trust was five years in the making. “It’s international cooperation, but it’s not like each partner is pursuing its own interest. Everybody is going after one big success of the mission as a whole,” says Fujimoto. “I really like this atmosphere.”